The invention relates to a method for determining the amount of ammonia which is stored in a storage container by a chemical bond to a solid storage medium, in particular a salt of an earth alkaline metal or the like, and which can be released by supplying heat in order to be supplied to a catalytic exhaust gas purification device of an internal-combustion engine. The invention relates as well to a corresponding storage container. Concerning the state of the art, reference is made, for example, to German Patent document DE 197 28 343 C1.
Within the scope of so-called “SCR technology” (selective catalytic reduction technology) for the catalytic purification of internal-combustion engine exhaust gases, particularly in motor vehicles, it is known to use ammonia. The ammonia can be carried along in the vehicle in various manners, for example, as a urea solution. However, the use of so-called solid storage devices is endeavored, in which ammonia is chemically bonded in a suitable storage medium, such as strontium chloride. In this case, the ammonia is released by supplying heat to the storage medium.
Since extraction reduces the amount of ammonia stored in such a solid storage device, it is endeavored to make it possible to easily determine the residual amount of ammonia still present in the solid storage device. This can be done, for example, by weighing the solid storage device when removed from the motor vehicle.
However, for a use in motor vehicles, there is needed a measuring method that is easier and, in particular, a measuring method that can also be carried out while the vehicle is driving.
This and other needs are met according to the invention by a method for determining the amount of ammonia, which is stored in a storage container by a chemical bond to a solid storage medium, in particular a salt of an earth alkaline metal or the like, and which is releasable by supplying heat in order to be supplied to a catalytic exhaust gas purification device of an internal-combustion engine. The storage medium functions or operates as a dielectric of an electric capacitor. The stored amount of ammonia is inferred from the capacity of this capacitor.
An advantageous storage container for ammonia, by which this method can be implemented, is characterized in that the wall of the storage container functions or acts as a first electrode of an electric capacitor, and in that at least one additional electrode of this capacitor is provided within the storage container and is arranged such that at least a partial amount of the storage medium forms the dielectric of this capacitor.
It was recognized that, if the storage medium is situated directly between two appropriately shaped electrodes, a dielectric can act with respect to these electrodes when an electric voltage or voltage difference is applied to the two electrodes, so that an electric capacitor will then be present. Furthermore, it was recognized that, while the electric voltage is essentially constant and the geometrical dimensions are constant, the (electric) capacity of this capacitor will change as a function of the charging degree of the storage medium, i.e. as a function of the amount of ammonia stored in the storage medium. A simple connection can thereby be established between the basically measurable capacity of the thus formed capacitor and the charging degree of the storage medium, preferably by experimental determination; for example, in that the storage medium is weighed at different charging degrees and the respective pertaining capacity is determined. When then later a storage container with this amount of storage medium and the same electrodes is installed in the motor vehicle, as required, the capacity of the thus formed capacitor can be determined, and by way of the previously determined known connections, the degree of charging of the storage medium can be inferred.
For example, two suitably designed electrodes can be provided within a storage container, and the storage medium can be provided between these electrodes. The two electrodes, for example, can be arranged in a circular-cylindrical storage container close to the two faces of the hollow cylinder. However, for the purpose of an advantageous combination of functions, it is particularly advantageous for the housing of the storage container, which receives the storage medium, itself to form one of the two electrodes while the, or generally another, electrode is provided, for example, centrally within the storage medium. Also in the case of such a further development, in a preferred embodiment, the storage container may have an essentially cylindrical shape, in whose cylinder axis the second electrode is arranged, or whose cylinder axis is one of the two electrodes and whose generated surface is the other of the two electrodes.
However, more than two electrodes may also be provided, whereby several capacitors are formed, between which a respective partial amount of the storage medium is situated. It thereby becomes possible to individually determine the charging of this or each partial amount. For example, in a cylindrical storage container, whose wall surrounding the storage medium acts as a first electrode of a first capacitor, several cylindrical electrodes can be arranged coaxially to one another in the interior of the storage container and thus quasi in the storage medium, resulting in a quasi series connection of several capacitors. In this case, it can be provided that the capacity of each individual capacitor can be determined individually, from which a possibly different charging of the storage medium can be determined in different areas of the storage container interior.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.